The invention relates to the use of surfactants as plasticizers in water-based polymer coating compositions to reduce or eliminate the need for conventional cosolvents and to reduce the overall volatile organic compounds (VOC""s) of the coating composition.
Both legislative and marketplace developments are pushing for reduced volatile organic emissions in a variety of industries. In an increasing number of industries, aqueous coating compositions continue to replace solvent-based coating compositions in efforts to significantly reduce volatile organic emissions. A variety of paints, inks, sealants and adhesives, for example, which were previously formulated with organic solvents are now formulated as aqueous compositions. Emissions from coatings compositions commonly result from volatile organic compounds (VOC""s) in the compositions. The amounts of VOC""s in a coating composition are expressed in grams per liter (g/l).
While the move from organic solvent-based to aqueous compositions brings environmental, safety and health benefits, aqueous coating compositions must still meet or exceed the performance standards expected from solvent-based coating compositions. The coatings or films must form at ambient temperatures (35xc2x0 to 160xc2x0 F.), yet have good performance properties after curing. For example, a coating composition should exhibit good print and block resistance and yield good adhesion and tensile properties. Once cured, most applications require that the coating be unaffected by environmental conditions such as water, humidity, and end-use temperature fluctuations.
Aqueous coating compositions may contain upwards of ten to twenty components which are generally identified by their function. For example, in addition to a resin or resins (also called latexes or binders), an aqueous coating composition may have pigments, extenders, antisettling agents, dispersants, surfactants (such as wetting agents, defoamers, and antifoamer), rheology modifiers, coalescing solvents, plasticizers, water, glycols, catalysts, biocides, crosslinkers, and colorants. Glycols are components added for freeze-thaw resistance, wet edge properties and as aids in low temperature coalescence. Representative glycols used for these purposes include ethylene glycol and propylene glycol. Because the glycols generally evaporate at ambient conditions, they contribute to VOC""s found in aqueous coating formulations. A typical contribution to VOC""s by glycols would be 100 to 200 grams per liter. Glycols are one of the first components aqueous coating manufacturers seek to decrease or eliminate in an effort to reduce emissions. However, the resulting coating may then suffer in the desired properties of low temperature coalescence, freeze/thaw resistance and wet edge.
Cosolvents (also known as coalescing solvents) are commonly employed in aqueous compositions to aid in film formation (or knitting-together) of hard latex particles. This hardness can be measured in terms of the starting film-formation temperature or of the glass transition temperature of the manufactured latex solid. As drying occurs, the cosolvents evaporate from the coating and the glass transition temperature of the coating approaches that of the starting resin. The addition of cosolvents enables the coating to behave like a softer film-forming material during drying and then perform as a harder, resistant film after drying. Examples of cosolvents include aliphatic and aromatic hydrocarbons and oxygenated solvents, such as alcohols, ketones and glycol ethers A typical amount of cosolvent ranges from 50 to 300 or more grams per liter of coating composition. Coating compositions based on cosolvents are described, for example, in Paint Handbook, 1-12 to 1-24 (Harold B. Crawford and Beatrice E. Eckes eds., 1981), incorporated here by reference. Because cosolvents present in the coating formulation contribute considerably to VOC""s content and tend to contribute odor to the coating, cosolvents are becoming more undesirable in aqueous coatings. However, in many aqueous coating systems, the elimination of the cosolvent(s) result in either lack of film formation or such poor film formation that the coating has poor appearance and poor resistance properties. In some cases, plasticizers may be added to the aqueous coating formulation to replace some or all of the cosolvents. Typically, plasticizers are organic compounds that do not significantly evaporate at ambient conditions but remain in the coating. Examples of typical plasticizers would be chemicals from the phthalate, adipate, and benzoate families. They soften the polymer and are used to impart flexibility to an otherwise hard and brittle polymer. However, plasticizers, especially at high levels, can have deleterious effects on coating performance properties. Because the coating remains soft, it can have poor block and print resistance, poor stain resistance and a tacky feel.
Many of the components discussed above used to formulate waterborne coatings have small amounts of volatile compounds present along with the components. Some examples are the solvents that colorants are dispersed in, the solvents that catalysts are dispersed in, and even the glycols or solvents that are present in many commercial surfactants, biocides, defoamers, or rheology modifiers. For example, the low level of VOC calculated for the formulation used in Examples 1-6 below (11 g/l) arises from a particular component; the solvents present in the cobalt catalyst employed.
Surfactants are commonly used in coating formulations to improve wetting of the substrate by the coating, and wetting of the pigment by the resin. They can also improve formulating latitude by preventing shocking of the coating composition as various components are added and can increase the service life of the coating by increasing shelf stability. Typically, low levels of surfactants are used to accomplish these goals and mixtures of surfactants may be employed to impart one or more of the properties listed above. Surfactants are not generally volatile materials under ambient conditions and remain in the coating during the drying process. However, at the low concentrations typically used, little effect on polymer hardness or coating performance is observed. If too much surfactant is used in the aqueous coating composition, the wet coating could exhibit excessive foaming and poor thickening efficiency with thickeners while the cured coating could have problems with water sensitivity, poor exterior durability and poor block, stain and print resistance. Thus, surfactants are typically used in the lowest amounts necessary to achieve their beneficial properties while avoiding any detrimental effects.
A discussed above, a need exists to reduce or eliminate VOC""s from aqueous coating compositions without effecting wet coating or end-use performance properties. The invention answers that need.
It has been discovered that surfactants can be employed at levels similar to those used by conventional cosolvents and function as a plasticizers in the coating, thus eliminating the need for conventional cosolvents and eliminate the VOC""s associated with the cosolvents. Preferred surfactants of the invention can be used to plasticize the coating, aid in low temperature film formation and contribute to the typical properties associated with surfactants in coatings while not contributing deleteriously to the final balance of properties in the finished coating.
The invention relates to an improved water-based polymer coating composition comprising a polymer resin, water and surfactant. As an improvement over previous water-based polymer coating compositions an anionic and/or nonionic surfactant is present in an amount effective to plasticize the coating formed from the composition. A water-based polymer coating composition of the invention is also substantially free from cosolvents. The invention also relates to a method of coating a substrate and a substrate which has been coated with a coating formulation of the invention.
The invention relates to an improved water-based polymer coating composition comprising a polymer resin, water and surfactant. An anionic and/or nonionic surfactant is present in an amount effective to plasticize the coating formed from the composition. This improves prior water-based coating compositions by reducing or eliminating the cosolvent and reducing the amount of VOC""s in the coating composition. A water-based polymer coating composition of the invention is also substantially free from cosolvents. Preferably, the VOC of the coating is less than 250 g/l, more preferably less than 50 g/l, even more preferably less than 25 g/l and most preferably VOC""s are eliminated.
Aqueous emulsion polymers or latexes in both clear and pigmented form are well-known. Examples of their uses include interior and exterior architectural coatings, general metal coatings, adhesives, and the like. Examples include the aqueous coating compositions described in Technology of Paints, Varnishes and Lacquers, Robert E. Krieger Publishing Co., Huntington, N.Y. 1974, U.S. Pat. Nos. 5,002,998 and 5,185,397 and in GB Patent No. 2,206,591, all of which are incorporated here by reference. Water-based polymer coating compositions conventionally contain from about 10 to about 40% by volume of solids.
Synthetic latexes are well known and can be made by emulsion polymerization techniques from styrene-butadiene copolymer, acrylate resins, polyvinyl acetate, and similar materials. For example, latexes can be formed by aqueous emulsion polymerization of ethylenically unsaturated monomers such as styrene, butyl acrylate, methyl methacrylate, vinyl acetate, vinyl 2-ethylhexanoate, acrylic acid, acrylonitrile, glycidyl methacrylate, 2-hydroxyethyl acrylate and the like. Preferred latexes for use in the invention include those described in U.S. Pat. No. 5,539,073 and copending application Ser. Nos. 08/861,431; 08/061,433; No. 60/047,324; 08/861,437; 08/861,436, all of which are incorporated here by reference.
Water-based polymer coating compositions may comprise pigments (organic or inorganic) and/or other additives and fillers known in the art. For example, a latex paint composition may comprise a pigment and one or more additives or fillers used in latex paints. Such additives or fillers include, but are not limited to, leveling, rheology, and flow control agents such as silicones, fluorocarbons, urethanes, or cellulosics; extenders; curing agents such as multifunctional isocyanates, multifunctional carbonates, multifunctional epoxides, or multifunctional acrylates; reactive coalescing aids such as those described in U.S. Pat. No. 5,349,026 (which are incorporated here by reference); flatting agents; pigment wetting and dispersing agents and surfactants; ultraviolet (LV) absorbers; UV light stabilizers; tinting pigments; extenders; defoaming and antifoaming agents; anti-settling, anti-sag and bodying agents; anti-skinning agents; anti-flooding and anti-floating agents; fungicides and mildewcides; corrosion inhibitors; thickening agents; plasticizers; reactive plasticizers; drying agents; catalysts; crosslinking agents; or coalescing agents. Specific examples of such additives can be found in Raw Materials Index, (published by the National Paint and Coatings Association, 1500 Rhode Island Avenue, NW, Washington, D.C. 20005), which is incorporated here by reference.
As discussed above, low levels of surfactants have conventionally been used in water-based polymer coating composition for their surfactent properties. Advantageously, it has been discovered that surfactants can be employed at levels similar to those of conventional cosolvents and function as a plasticizers in the coating. This eliminates the need for conventional cosolvents and significantly reduces, or preferably eliminates, the VOC""s in the coating composition, particularly VOC""s associated with the cosolvents. Further, problems such as blocking, poor print and stain resistance associated with plasticizers, are eliminated.
Any anionic or nonionic surfactant, as well as mixtures, may be used in a water-based polymer coating composition of the invention. The surfactant is present in an amount effective to plasticize a coating formed from the composition, preferably ranging from about 3 to about 10% by weight of the dry polymer. Preferred anionic surfactants include alkali metal or ammonium salts of alkyl, aryl or alkylaryl sulfonates, sulfates, phosphates. More preferably, the anionic surfactant is selected from sodium lauryl sulfate, sodium octylphenol glycolether sulfate, sodium dodecylbenzene sulfonate, sodium lauryldiglycol sulfate, ammonium tritertiarybutyl phenol and penta- and octa-glycol sulfonates, sulfosuccinate salts such as disodium ethoxylated nonylphenol half ester of sulfosuccinic acid, disodium n-octyldecyl sulfosuccinate, sodium dioctyl sulfosuccinate, and mixtures thereof AEROSOL 18 surfactant, a 35% solution of disodium N-octyldecyl sulfosuccinimate in water and AEROSOL OT-75 surfactant, a 75% solution of sodium dioctyl sulfosuccinate in water, both available from Cytec Industries, Inc. are preferred anionic surfactants.
Preferably the nonionic surfactant is a polyether nonionic surfactant, more preferably, an alkyl polyglycol ether, an alkyl phenol polyglycol ether or a mixture thereof, such as those disclosed in U.S. Pat. Nos. 4,912,157 and 5,554,675 the disclosures of which are incorporated herein by reference. Preferred alkyl phenol polyglycol ethers include ethoxylation products of octylphenol, nonylphenol, diisopropyl phenol, triisopropyl phenol or mixtures thereof. Preferred alkyl polyglycol ethers include ethoxylation products of lauryl alcohol, oleyl alcohol, stearyl alcohol or mixtures thereof. Preferred nonionic surfactants include the TERGITOL 15-S-40 and TERGITOL NP-40 surfactants available from Union Carbide. TERGITOL 15-S-40 surfactant (CAS #68131-40-8) is a reaction product of a mixture of 11-15 carbon, linear secondary alcohols and ethylene oxide. TERGITOL NP-40 surfactant is the reaction product of a nonylphenol and about 40 moles of ethylene oxide.
Most preferably the alkyl polyglycol ether is selected from compounds of the formula:
Rxe2x80x94Cxe2x89xa1Cxe2x80x94R1 
wherein R and R1 are each selected from straight and branched alkyls having from 1 to 15 carbon atoms and wherein at least one of R and R1 contains from 1 to 3 hydroxyl groups, and the H of each hydroxyl group is independently unsubstituted or substituted with a substituent of the formula
(CH2xe2x80x94CH2xe2x80x94O)nH 
or
(CH2xe2x80x94CH(CH3)xe2x80x94O)mH 
wherein n and m each range from 1 to about 50 and wherein the total of n and m is less than about 60. These compounds are known in the art and are available commercially from Air Products and Chemicals, Inc. under the trade name SURFYNOL(copyright), including the SURFYNOL(copyright) 104 series, SURFYNOL(copyright) 420, 440, 465 and 485, also known as the SURFYNOL(copyright) 400 series.
SURFYNOL(copyright) 104 has the following formula: 
The SURFYNOL(copyright)400 series have the following general formula: 
wherein the sum of m and n ranges from about 1 to about 30, with SURFYNOL(copyright)420 having 1.3 moles of ethylene oxide, SURFYNOL(copyright)440 having 3.5 moles of ethylene oxide, SURFYNOL(copyright)465 having 10 moles of ethylene oxide and SURFYNOL(copyright)485 having 30 moles of ethylene oxide.
Upon formulation, a coating formulation of the invention containing a polymer or waterborne polymer composition may then be applied to a variety of surfaces, substrates, or articles, e.g., paper, plastic, steel, aluminum, wood, gypsum board, concrete, brick, masonry, or galvanized sheeting (either primed or unprimed). The type of surface, substrate, or article to be coated generally determines the type of coating formulation used. The coating formulation may be applied using means known in the art. For example, a coating formulation may be applied by spraying, brushing, rolling or any other application method to coat a substrate. In general, the coating may be dried by heating but preferably is allowed to air dry. Advantageously, a coating employing a polymer of the invention may be thermally or ambiently cured. As a further aspect, the invention relates to a shaped or formed article which has been coated with a coating formulation of the invention.
The following examples are intended to illustrate, not limit, the invention. The examples of various coating compositions of the invention use the following materials not described above:
TAMOL 1124 is a dispersant sold by Rohm and Haas.
ROPAQUE OP-62LO is an opaque polymer sold by Rohm and Haas.
ACRYSOL RM-5 is a rheology modifier (thickener) sold by Rohm and Haas.
COBALT HYDROCURE II (cobalt neodecanoate, 45% solids) is a cobalt drier sold by Mooney Chemical, Inc., Cleveland, Ohio.
FOAMASTER AP and VF are defoamers sold by Henkel Corporation, Ambler, Pa.
TRITON CF-10 is a surfactant sold by Union Carbide Chemicals and Plastics, Corp.
CELLOSIZE 4400H is a rheology modifier sold by Union Carbide, Bound Brook, N.J.
DOWICIL 75 is a preservative sold by DOW Chemical Company, Midland, Mich.
TI-PURE R-900, R-746 and R-760 are titanium dioxide pigments sold by Du Pont, Wilmington, Del.
OMYACARB UF is a calcium carbonate pigment, sold by Omya Inc., Proctor, Vt.
RHEVOIS CR2 is a rheology modifier sold by Allied Colloids, Suffolk, Va.
TAFIGEL PUR 45 is a rheology modifier distributed by King Industries, Norwalk Conn.
TREM LF-40 is a polymerizable surfactant sold by Henkel Corporation, Ambler, Pa.
HITENOL HS-20 is a polymerizable surfactant available from Daiichi Kogy Seiyaku.
POLYM N-G is a poly(alkyl ethylenimine) available from the BASF Corporation.
TERGITOL 15-S-40 and TERGITOL NP-40 are surfactants available from Union Carbide Chemicals and Plastics, Corp.
AEROSOL 18 and AEROSOL OT-75 are anionic surfactants available from Cytec, Industries, Inc.
PROXEL GXL is a preservative sold by Zeneca Biocides, Wilmington, Del.
DURAMITE is a calcium carbonate pigment sold by ECC America, Atlanta, Ga.
NYTAL 300 is a talc pigment sold by RT Vanderbilt, Norwalk, Conn.
The following methods were used to evaluate the coatings prepared according to the invention.
Constant Temperature and Humidity Room:
Films were prepared and film measurements were conducted at ASTM standard conditions for laboratory testing of 73.5xc2x13.5xc2x0 F. (23xc2x12xc2x0 C.) and 50xc2x15% relative humidity.
Minimum Film Forming Temperature (MFFT):
Minimum film forming temperature (MFFT) is determined by casting a wet latex film with a 4-mil applicator on an MFFT bar set at a temperature range in which the film will coalesce during drying, visually observing the film on the MFFT bar after 30 minutes, and recording the temperature at which the film appears to have coalesced and is free of cracks and film defects.
Tensile:
Tensile tests are performed in the CTH room on a on a United Tensile Tester, which has constant rate of elongation machine. Film samples are obtained by casting the sample on release paper with a 7 mil bird bar, drying the film for the desired time at the stated conditions, and cutting a dogbone-shaped thin-film sample with a 1xe2x80x3 wide die. The film is measured for film thickness, mounted in the tensile tester grips and tested at a cross head speed of 1xe2x80x3/minute using a 5 lb-force load cell. Ten samples are run and the five samples with the greater breaking stress are averaged for all tensile values reported according to ASTM
Glass Transition:
Onset and midpoint temperatures were determined on film samples using a differential scanning calorimeter (DSC) in a nitrogen atmosphere at heating rate of 20xc2x0 C./minute. Values quoted are from the reheat curve.
Paint Viscosity:
Paint viscosity (in Krebs Units) was measured after 24 hours using a Krebs-Stormer viscometer.
Gloss:
Gloss was measured on 6 mil (wet) thick films cast on Leneta 2B opacity paper after 24 hours using a Micro-Tri-Glossmeter by BYK-Gardner according to ASTM method D 523 Test Method for Specular Gloss.
Blocking Resistance:
Blocking resistance was determined using 6 mil (wet) films on Leneta 2B opacity paper according to ASTM 4946 Test Method for Blocking Resistance of Architectural Paints using 1 psi pressure after film dried to designated times. Heated block resistance was determined in a forced air oven at 120xc2x0 F. with the painted surfaces face-to-face under 1 psi pressure for 30 minutes. The tests were numerically rated where a rating of 10 represents 100% pass where painted surfaces lift apart with no noise; a rating of 9-4 represents the degree of noise when painted surfaces are separated; a rating of 3-1 represents degree of destruction of the painted surfaces when the two surfaces are separated; and a rating of 0 represents 100% fail where the painted surfaces flow completely together and complete destruction of the films occurs upon separation.
Print Resistance:
Print resistance was determined using 6 mil (wet) films on Leneta 2B opacity paper according to ASTM D 2064-91 Test Method for Print Resistance of Architectural Paints using a 4 pound weight placed on top of a #6 black rubber stopper which was placed on four layers of cheesecloth after film dried to designated times. Heated print resistance was determined in a forced air oven at 120xc2x0 F. with folded cheesecloth (as above) under a pressure of 4 pounds for 30 minutes. The tests were numerically rated as per ASTM D2064-91.
Scrub Resistance:
Scrub resistance was determined following ASTM D2486 Test Method for Scrub Resistance of Architectural Coating. The coating is applied at 7 mil wet on Scrub Test Charts Form P121-10N and allowed to dry for the specified period of time. The panel is placed in a Gardco Scrub Machine, Model D-10V, 10 g of Standardized Scrub Medium (abrasive type) for ASTM D2486 and D3450 is placed on the scrub brush, the panel is wet with 5 ml DI water, the test machine counter is zeroed, and the test is run at the maximum test speed on the machine. After each 400 cycles before failure, the brush is removed and 10 more g of scrub medium is added evenly on the bristles, the brush is replaced, 5 ml of DI water is placed on the panel and the test is continued. The test is run to failure. Failure is defined as the number of cycles to remove the paint film fully in on continuous line across the width of the shim.
Wet Adhesion Test:
This procedure tests the coatings adhesion to an aged, alkyd substrate under wet, scrubbing conditions. This procedure is described in xe2x80x9cVYNATE(trademark) (Union Carbide Chemicals and Plastics Corporation)xe2x80x94Vinyl Emulsion Vehicles for Semigloss Interior Architectural Coatings,xe2x80x9d M. J. Collins, et. al., presented at the 19th Annual xe2x80x9cWater-Borne High-Solids and Powder Coating Symposiumxe2x80x9d, Feb. 26-28, 1992, New Orleans, La., USA.
A ten-mil drawdown of a commercial gloss alkyd paint is made on a xe2x80x9cLenetaxe2x80x9d scrub panel (adhesion varies from alkyd to alkydxe2x80x94a Glidden Industrial Enamel was used.) The alkyd film is allowed to age one week at ambient conditions, then baked at 110xc2x0 F. for 24 hours, and then aged at least one more week at ambient conditions. A seven-mil drawdown of the test paint is then made over the aged alkyd and allowed to air dry three days. (In order to differentiate between samples that pass this test, dry times may be shortened. Seven days is a common period, and occasionally 5 hours dry time is used. Constant temperature/humidity conditions, 72xc2x0 F./50%, are normally used for drying.) The test paint is then cross-hatched with a razor and submerged in water for 30 minutes. The paint film is inspected for blistering and scratched with the fingernail to gauge the adhesion. While still wet, the panel is placed on a xe2x80x9cGardnerxe2x80x9d scrub machine. Ten ml of five percent xe2x80x9cLAVA(trademark)xe2x80x9d soap slurry are added, and the Nylon scrub brush (WG 2000NB) is passed over the scored paint film area. Water is added as needed to keep the paint film wet (flooded). The number of brushing cycles for initial peel and ten percent peel are noted. The number of cycles for complete removal of the film is often noted also.
Stain Test:
Apply test paint with 6 mil draw down bar. After draw down, allow the test paint(s) to cure for 21 days at 72xc2x0 F.xc2x12 and relative humidity of 50%xc2x12. Expose the paint film to black shoe polish, catsup, TOP JOB(copyright), crayon, grape juice, red KOOL AID(copyright) and PINE SOL(copyright) for a total time of 5 hours. Expose the paint film to mustard, coffee and nigrosine for a total of 30 minutes. Expose the paint film to red ink and blank ink for a total of 5 minutes. Expose the paint film to iodine for 30 seconds. Cover all stains with 1 xc2xd inch watch glass for the specified exposure time. Remove the stain with a soft cloth soaked in a mild detergent solution. Rate the degree of stain using a scale of 1-5 with 1=no stain and 5=extreme stain of film destroyed.
Low Temperature Coalescence:
Low temperature coalescence was determined in accordance with ASTM D3793-89.